Low attenuation optical fiber

ABSTRACT

A low attenuation optical fiber which falls within 2-14 ps/nm/km in absolute value of dispersion over the full wavelength range of 1530-1565 nm and no more than 0.25 dB/km of transmission loss at 1550 nm of wavelength at ordinary temperature and relative humidity, and still remains no more than 0.25 dB/km of transmission loss at 1550 nm or 1520 nm after its being long-enough exposed under ordinary atmospheric pressure consisting substantially of hydrogen; and which further comprises a polarization mode dispersion (PMD) of no more than 0.5 ps/km at a wavelength of 1550 nm and a loss increase of no more than 40 dB/m in a bending diameter of 20 mm.

FIELD OF THE INVENTION

The present invention relates to low-attenuation optical fibers suitableform wavelength-division-multiplexing (WDM) optical transmissionsystems.

BACKGROUND OF THE INVENTION

Vigorous studies have been given on techniques to increase the capacityof optical-fiber transmission with optical fibers.

It is believed that a growth of optical transmission capacity requiresthe optical fibers for the optical transmission to enable single-modetransmission at the operating wavelength, because the groupings ofdifferent speeds of optical signals in various modes can induce modedispersion inevitably in the propagation through an optical fiber. As aresult, the signal waveforms can decay or warp.

Consequently, the single-mode fiber (SMF) was started in use, having azero-dispersion wavelength around 1.3 μm. At the zero-dispersionwavelength, the fiber was able to have a transmission distance of scoresof kilometers, and a transmission capacity of hundreds of Mbps (megabitsper second).

In the meantime, the least transmission loss in optical fiber takesplace at 1.55 μm of wavelength, where a dispersion-shifted fiber (DSF)with a zero-dispersion wavelength of 1.55 μm or thereabout wasdeveloped. This optical fiber enabled the optical transmission opticaltransmission with a capacity of several gigabits per second around 1.55μm of wavelength. The same single-mode fibers were laid in long-distanceoptical transmission routes each with a capacity of several G bit/s in a1.55 μm wavelength band.

In the latter half of the 1980s it was discovered that transmissionloses would increase in an optical fiber in which hydrogen molecules,from a hydrogen gas (H2) trapped in the cable, had been broken. Onanalysis, the loss increase was assignable to absorption peaks in thetransmission-loss spectrum, which hydrogen molecules had induced in theoptical fiber. Hydrogen-induced absorption peaks emerge around 1.24 μm,and at 1.52 μm and on the longer-wavelength side. The absorption peaksat 1.52 μm and the longer wavelength were seen to have an adverse impacton the optical transmission around 1.55 μm, firsthand, for instance, asdescribed in ECOC '86, pp7-10, by Ogai et al.

Concurrently, in terms of 1.31 μm transmission SMF and 1.55 μmtransmission DSF, assorted R&D approaches were made to prevent thehydrogen-induced loss increase, from the aspect of fabrication techniqueor fiber coating material. For example, optical-fiber cables forterrestrial application were usually filled with a filling compound soas to reduce the amount of trapped gaseous hydrogen. Accordingly, nohydrogen-proofing techniques were explored (see e.g.,Bellcore-GR-20-Core issue 2, Jul. 1998, Section 6.6.9).

In the recent years, in search for more capacities of opticaltransmission systems, the designs of wavelength division multiplexing(WDM) have been studied and developed, producing volumes of reports onoptimizing optical fibers for WDM transmission.

From the angle of evading four-wave mixing, the optical fibers for WDMoptical transmission systems are required to be unequipped with azero-dispersion wavelength in their operating wavelength bands. In thiscontext, a non-zero dispersion shifted fiber (NZDSF) has been developed,without any zero dispersion in the operating wavelength band. Ingeneral, NZDSFs are required to have even more complicatedrefractive-index (RI) profiles, than those of SMFs or DSFs, because theyneed to gear with additional requirements for a large effective corearea (Aeff), a reduced dispersion slope, etc. to provide forhigh-density WDM (DWDM) optical transmission.

Complicated RI profile designs of NZDSFs accompany a propensity toinduce minute glassmaking flaws in optical fibers, along with irksomeprocess control.

Although NZDSFs are in use to cover a broad wavelength band including1.55 μm, no hydrogen-proof treatment techniques were then disclosed tothe public.

In the recent years, cables to shroud optical fibers have structurallybeen reviewed and improved. In fact, optical fiber cables are shiftingin great numbers from a compound-filled type to a dry-core type whichcontains a water absorbent material in the cable instead of a fillingcompound. The filling-compound free cable fabrication is far lesstoilsome (not required to wipe clean the cables). Also the filled cablescould hardly be enhanced in fire resistance, but filling-compound free(dry-type) cables can readily be attached with enhanced fire resistance.A sample dry-core type of optical fiber cable is described in U.S. Pat.No. 5,422,973.

The dry-core type contains a water absorbent material in the cable toblock out lengthwise, water penetration, which contacts wet, swells anddarns off the water. But then, the water absorbent material has anaction to lead in ambient humidity (moisture), even without any cabledamage or opening, and poses a threat of allowing the trapped wet(absorbed ambient moisture) to react with component metals inside thecable, where hydrogen ions emerge. Accordingly, even optical fibers forterrestrial cables need to be considered about their hydrogen-prooftreatment.

For instance, U.S. Pat. No. 6,131,415 describes an optical fiber with athought of hydrogen-proof performance (hydrogen resistance), and atechnique for suppressing the hydroxyl-ion concentration in an opticalfiber to reduce the transmission loss at 1385 nm. In particular, thepresent patent owner, Lucent Technologies, discloses in “CatalogAllwave”, certain information about a required design concept ofhydrogen-proof performance (hydrogen resistance) of optical fibers formetropolitan use.

Moreover, U.S. Pat. Nos. 5,838,866 and 6,128,928 each describe anoptical fiber with a thought of hydrogen resistance. Each fiber isdesigned to be equipped with hydrogen resistance by making the (inner)clad contain germanium to a degree to raise its refractive index, not insubstance. However, U.S. Pat. No. 6,131,415 remarks on no more than atechnique for suppressing the loss increase at 1385 nm, arising out ofabsorption peaks of hydroxyl ions, without any remarks on the lossincrease at 1520 nm, due to the absorption peaks of hydrogen molecules.

Moreover, none of the techniques in U.S. Pat. Nos. 6,131,415, 5,838,866and 6,128,928 involve any additives to bring on substantial shifts inthe refractive-index profile in each clad region. Thus, these threepatents are expected to aim for characteristic improvements in “SMF” orthe equivalent, but not NZDSF for WDM optical transmission systems.

SUMMARY OF THE INVENTION

A purpose of the present invention is to provide low-attenuation fibers,or specifically, a low-attenuation fiber which comprises:

a dispersion D of 2.0 to 14.0 ps/nm/km in a wavelength range of 1530 to1565 nm in absolute value,

a transmission loss of 0.25 dB/km or less under the standard atmosphericconditions, wherein

the transmission loss at 1550 nm and/or 1520 nm in wavelength is madenot to exceed 0.25 dB/km, as a result of being exposed to an atmospherecomposed substantially of hydrogen under the ordinary atmosphericpressure at ordinary temperature for a certain period. The standardatmospheric conditions refer to definitions in JISC0010, Section 5 andare made up of ordinary temperature (25±10° C.), ordinary relativehumidity (25-75%) and ordinary atmospheric pressure (86-106 kPa). Inaddition, the certain period need to be no less than a time lapse inwhich hydrogen penetrates the midmost/core of an optical fiber. Ineffect, the hydrogen penetration is represented by at least 0.03 dB/kmof loss increase around 1.24 μm.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a complicated refractive-index (RI) profile for a lowattenuation optical fiber of one embodiment in the present invention.

FIG. 2 is a second complicated refractive-index (RI) profile for a lowattenuation optical fiber of another embodiment in the presentinvention.

Legend:

1 Core region

2 1st annular region

3 2nd annular region

4 3rd annular region

5 Clad

DETAILED DESCRIPTION

The present invention is designed to provide hydrogen-proof opticalfibers with moderate dispersions in a wavelength band between 1530 and1565 nm and transmission losses remaining almost constant over time.claim 1 of the present invention recites a low attenuation optical fiberwhich falls within 2-14 ps/nm/km in absolute value of dispersion overthe wavelength range of 1530-1565 nm and no more than 0.25 dB/km oftransmission loss at 1550 nm of wavelength at ordinary temperature andrelative humidity, and still remains no more than 0.25 dB/km oftransmission loss at 1520 nm of wavelength even after its beinglong-enough exposed under ordinary atmospheric pressure consistingsubstantially of hydrogen.

Claim 7 of the present invention recites a low attenuation optical fiberwhich falls within 2-14 ps/nm/km in absolute value of dispersion overthe wavelength range of 1530-1565 nm and no more than 0.25 dB/km oftransmission loss at 1550 nm of wavelength at ordinary temperature andrelative humidity, and still remains no more than 0.25 dB/km oftransmission loss at 1520 nm of wavelength even after its beinglong-enough exposed under ordinary atmospheric pressure consistingsubstantially of hydrogen.

Claim 2 of the present invention recites a low attenuation optical fiberbased on claim 1 or claim 2, which further comprises a dispersion slopeof no more than 0.15 ps/nm²/km over the wavelength band of 1530-1565 nm,a PMD of no more than 0.5 ps/km at a wavelength of 1550 nm and a lossincrease of no more than 40 dB/m in a bending diameter of 20 mm.

Claim 3 of the present invention recites a low attenuation optical fiberbased on claim 1, which further comprises 90 μm² of Aeff.

Claim 4 of the present invention recites a low attenuation optical fiberbased on claim 1, which further comprises a dispersion slope of0.04-0.08 ps/nm²/km over the wavelength band of 1530-1565 nm, anabsolute value of dispersion of 6-10 ps/nm/km over the wavelength bandof 1530-1565 nm and an Aeff of 40-70 μm².

Building blocks of the present invention as claimed up to now can makeoptical fibers suitable for the WDM optical transmission in a wavelengthband of 1530-1565 nm.

The numerical limitations in the Claims are rounded in a method asauthorized by ASTM E29.

An embodiment of the present invention is explained with drawings fromnow onward.

FIG. 1 is displays a complicated refractive-index (RI) profile for alow-attenuation optical fiber of the present invention. FIG. 1 shows themidmost (core) region 1 and a clad 5, between which a first annularregion 2 and a second annular region 3 are located. The maximumrefractive indices (RI) in the core and second regions 1 and 3 arelarger than in the clad 5. The first annular region 2 is lower inrefractive index than clad 5.

FIG. 2 shows a second complicated refractive-index (RI) profile which anoptical fiber of the embodiment may have. The profile in FIG. 2 has themidmost (core) region 1 and a clad 5, between which a first annularregion 2, a second annular region 3 and a third annular region arelocated. The maximum refractive indices (RI) in the core and secondregions 1 and 3 are larger than in the clad 5. Refractive indices in thefirst and third regions 2 and 4 are lower than in the clad 5.

Notably, the RI profiles in FIGS. 1 and 2 are simply samples to whichlaw-attenuation fibers of the present invention are not limited, in RIprofile, provided that any RI profiles regarded as accordant with thepresent invention can be accepted.

In this embodiment, optical fibers are designed to have an absolutevalue of dispersion=2 ps/nm/km≦|D |≦14 ps/nm/km over the wavelength (λ)range of 1530 nm≦λ≦1565 nm. It is because, in the wavelength (λ) rangeof 1530 nm≦λ≦1565 nm, a four wave mixing impact is undesirably large atan absolute value of dispersion: |D| less than 2 ps/nm/km; and,cumulative dispersions might unfavorably be combined to makehigh-density WDM optical transmission impracticable in the designedtransmission routes (systems), at an absolute value of dispersion: |D|more than 14 ps/nm/km.

In contrast to conventional cases, an optical fiber of the presentinvention and as long-enough exposed under a ordinary atmosphericpressure consisting substantially of hydrogen at ordinary temperaturecan never exceed 0.25 dB/km in transmission loss at 1550 nm. Thedesigned characteristic results from a fact of optical fibers withtransmission losses in excess of 0.25 dB/km regarded as undesirable at1520 or 1550 nm for long-distance WDM transmission, even with theirhaving been long-enough exposed to a hydrogen atmosphere. Here, theordinary atmospheric pressure consisting substantially of hydrogen meansto yield an effect equivalent to a 100% hydrogen atmosphere; anatmosphere of at 90% hydrogen is desirable, if the remainder, 10% orless is air. The long-enough exposed means a time lapse enough to allowhydrogen to reach the core of an optical fiber. Practically, this timelapse corresponds to when a loss increase of at least 0.03 dB/km isdetermined at 1.24 μm.

Considering the requirements for fiber characteristics in a wavelengthband of 1530-1565 nm, a range of dispersion of more than 15 ps/nm/km isundesirable, because of its making dispersions vary greatly fromwavelength to wavelength in WDM optical transmission; a polarizationmode dispersion (PMD) of more than 0.5 ps/km is undesirable, because ofits inducing a mass of polarization dispersion to disable WDM opticaltransmission; a bending loss increase of more than 40 dB/m isundesirable, because of its inducing variations in transmission loss inoptical fiber cables.

In addition, more than 90 μm² of Aeff in a wavelength range of 1530-1565nm is undesirable, because of its making the bending loss increase growin volume hardly allowing cabling the optical fibers. Less than 40 μm²of Aeff is undesirable, because of its being more likely to inducenonlinear phenomena. In particular, more than 70 μm² of Aeff, whichwould be a factor in disagreeing with other characteristic requirements,ought to be studied out, against discrete system specifications inpractice.

Here, the effective area (Aeff) is defined with reference to Opt. Lett.,Vol. 19, No. 4, pp 257-259 (Feb. 115,1994).

A slope of dispersion, which would preferably be minimized, could be afactor in disagreeing with other characteristic requirements, if reducedto less than 0.04 ps/nm²/km, and should be studied out, against discretesystem specifications in practice, as to choice to less than 0.04ps/nm2/km. Conversely, more than 0.08 ps/nm²/km of dispersion slope,with a propensity to risk the application to high-density WDM opticaltransmission, would preferably be studied out, against discrete systemspecifications in practice, as to choice to more than 0.08 ps/nm²/km.

In terms of absolute values of dispersion, a four-wave mixing effectwould arise below 6 ps/nm/km and cumulative dispersions would risk theapplication to high-density WDM optical transmission above 10 ps/nm/km.Design values of dispersion should preferably be set up , in view ofdiscrete virtual system specifications.

In conclusion, considering the above details, it is preferable to optfor 40-70 μm² of Aeff, 0.04-0.08 ps/nm²/km of dispersion slope range,and 6-10 ps/nm/km of absolute value of dispersion, in a wavelength bandof 1530-1565 nm.

To create optical fibers of the present invention with refractive index(RI) profiles in FIGS. 1 and 2, the fibers underwent a hydrogen-prooftreatment and hydrogen-proof test so as to demonstrate the presentinvention.

To name but a few, the hydrogen-proof treatments include a method ofsuperficially etching in-process performs in U.S. Pat. No. 6,131,415, amethod of treating optical fibers in a heated heavy-hydrogen atmosphere(Deutrium, D2) in European Patent Application No. 0673895A2. Introducingthe hydrogen-proof treatment into the production line, more safety andless process change are the most important items to be considered. Afterthe result in studying the condition of the hydrogen-proof treatment, wefound that the hydrogen-proof treatment under ordinary atmosphericpressure at ordinary temperature could provide sufficient hydrogenresistance for the NZDSFs.

In one method, a 3 km-long coated optical fiber was led and held, forabout 3 hours, in a treatment tank charged with D2 (heavy hydrogen) atalmost ordinary temperature. Notably, more than two hours of remainingin a D2 atmosphere will produce a substantially constant effect, subjectto a fiber length of 3 kilometers, where the hydrogen atmosphere doesnot need to be heated, but can produce a sufficient effect even atordinary temperature holding time in a hydrogen atmosphere is requiredto be longer as the fiber under treatment becomes longer.

In the hydrogen-proof test, a 3 km-long coated optical fiber was led andheld, for six hours, in a test chamber charged with H2 at ordinaryatmospheric pressure and ordinary temperature; and the test chamber wasrecharged with nitrogen or air at ordinary temperature and the fiber wastested for transmission loss after dozens of hours. Here, more than four(4) hours of holding time in a hydrogen atmosphere will produce analmost constant effect, subject to a fiber length of three (3)kilometers. However, the holding time in a hydrogen atmosphere isrequired to be longer as the fiber under treatment becomes longer.

Pre/post hydrogen-proof treatment transmission losses (dB/km) arepresented in Table 1, resulting from 20 sample pieces per embodiment,and consist of the discrete worst values. Note that measures (units) inthe table are as follows:

Dispersion: ps/nm/km

Dispersion slope: ps/nm²/km

PMD: ps/km

Loss increase, 20mm in bend diameter: dB/m

Aeff; μm².

Values of dispersion, dispersion slope are the maximums in 1530-1565 nm.Other values are taken at a wavelength of 1550 nm.

TABLE 1 Dispersion Bend 1.55 μm loss 1.52 μm loss RI Dispersion SlopePMD loss Aeff (dB/km) (dB/km) profile (ps/nm/km) (ps/nm²/km) ps/km(dB/m) (μm²) pre-test post-test pre-test post-test Embodiment 1 FIG. 111 0.05 0.03 1 53 0.213 0.224 0.220 0.228 Embodiment 2 FIG. 1 9 0.060.08 5 56 0.201 0.215 0.209 0.218 Embodiment 3 FIG. 1 7 0.06 0.09 6 560.205 0.217 0.213 0.224 Embodiment 4 FIG. 2 5 0.07 0.12 15 57 0.2090.221 0.218 0.227 Reference 1 FIG. 1 9 0.06 0.08 5 56 0.201 0.268 0.2090.396 Reference 2 FIG. 2 5 0.07 0.12 15 57 0.209 0.277 0.218 0.420

Table 1 reveals that hydrogen-proof optical fibers have been suppressedto be less than 0.25 dB/km in transmission loss, even after eachhydrogen-proof test in a hydrogen (D2) atmosphere. In contrast,References 1 and 2 without hydrogen-proof treatment, turn out to havemore than 0.25 dB/km in transmission loss after hydrogen-proof test.

Moreover, it is verified that hydrogen-proof optical fibers with otherRI profiles (e.g., single peaked, stepped or W pattern) than those inFIGS. 1 and 2, have incurred no increase in transmission loss in ahydrogen-proof test.

As unveiled in the above descriptions, the present invention hassuperior effects to facilitate the fabrication of optical fibers suitedto the WDM optical transmission in a wavelength band of 1530-1565 nm.

Since the above embodiments are described only for examples, the presentinvention is not limited to the above embodiments and variousmodifications or alterations can be easily made therefrom by thoseskilled in the art without departing from the scope of the presentinvention.

What is claimed is:
 1. A low attenuation optical fiber, comprising: alight transmitting region having a dispersion characteristic (D) of 2.0to 14.0 ps/nm/km in absolute value over a wavelength band of 1530 to1565 nm, and a transmission loss wavelength of 1550 nm which does notexceed 0.25 dB/km under standard atmospheric condition, wherein saidtransmission 1550nm does not exceed 0.25 dB/km after being exposed, fora predetermined period, to an atmosphere consisting substantially ofhydrogen under ordinary atmospheric pressure at ordinary temperature. 2.The low attenuation optical fiber according to claim 1, furthercomprising: a dispersion slope (S) of no more than 0.15 ps/nm/km over awavelength band of 1530 to 1565 nm; a polarization mode dispersion (PMD)of no more than 0.5 ps/km; and, a loss increase of no more than 40 dB/mat a wavelength of 1550 nm as coiled in a diameter of 20 mm.
 3. The lowattenuation optical fiber according to claim 1, further comprising: aneffective area (A eff) of no more than 90 μm2 at a wavelength of 1550nm.
 4. The low attenuation optical fiber according to claim 1, furthercomprising: a dispersion slope of 0.04 ps/nm/km to 0.08 ps/nm/km over awavelength band of 1530 to 1565 nm. a dispersion of 6 ps/nm/km 10ps/nm/km in absolute value, and an effective area of 40 μm2 to 70 μm2 ata wavelength of 1550 nm.
 5. The low attenuation optical fiber accordingto claim 1, further comprising: an effective area of no more than 90 μm2at a wavelength of 1550 nm.
 6. The low attenuation optical fiberaccording to claim 2, further comprising a dispersion slope of 0.04ps/nm/km to 0.08 ps/nm/km over a wavelength band of 1530 to 1565 nm; adispersion of 6 ps/nm/km 10 ps/nm/km in absolute value, and an effectivearea of 40 μm2 to 70 μm2 at a wavelength of 1550 nm.
 7. A lowattenuation optical fiber, comprising: a light transmitting regionhaving a dispersion characteristic (D) of 2.0 to 14.0 ps/nm/km inabsolute value over a wavelength band of 1530 to 1565 nm, and atransmission loss at a wavelength of 1550 nm which does not exceed 0.25dB/km under standard atmospheric conditions; wherein a transmission lossat a wavelength of 1520 nm which does not exceed 0.25 dB/km after beingexposed, for a predetermined period, to an atmospheric consistingsubstantially of hydrogen under ordinary atmospheric pressure atordinary temperature.
 8. The low attenuation optical fiber according toclaim 7, further comprising: a dispersion slope (S) of no more than 0.15ps/nm²/km over a wavelength band of 1530 to 1565 nm; a polarization modedispersion characteristic (PMD) of no more than 0.5 ps/km;and, a lossincrease of no more than 40 dB/km at a wavelength of 1550 nm as coiledin a diameter of 20 mm.
 9. The low attenuation optical fiber accordingto claim 7, further comprising: an effective area (Aeff) of no more than90 μm² at a wavelength of 1550 nm.
 10. The low attenuation optical fiberaccording to claim 7, further comprising: a dispersion slope of 0.04ps/nm²/km to 0.08 ps/nm²/km over a wavelength band of 1530 to 1565 nm; adispersion of 6 ps/nm/km to 10 ps/nm/km in absolute value; and aneffective area of 40 μm² to 70 μm² at a wavelength of 1550 nm.
 11. Thelow attenuation optical fiber according to claim 8, further comprising:an effective area of no more than 90 μm² at a wavelength of 1550 nm. 12.The low attenuation optical fiber according to claim 8, furthercomprising: a dispersion slope of 0.04 ps/nm²/km to 0.08 ps/nm²/km overa wavelength band of 1530 to 1565 nm; a dispersion of 6 ps/nm/km to 10ps/nm/km in absolute value, and an effective area of 40 μm² to 70 μm² ata wavelength of 1550 nm.